The invention relates to a method and an apparatus for straightening a workpiece, especially a crankshaft with a theoretical axis and at least on fillet running circularly about the axis, by exerting pressure with a tool that can move transversely of the axis and can be laid into the fillet, against at least one preselected section of the wall defining the fillet.
The fatigue strength of workpiece having circular rotating fillets, e.g., crankshafts, axle journals, turned-down shafts, or the like, is adversely affected particularly by the reduction of strength due to the shape of the material in the area of these fillets. It is therefore known to treat such workpieces by a method to be referred to hereinafter as "hard-rolling" and consists in pressing solid rollers with hydraulic jacks against the fillets, especially against walls defining curved fillets (DE-PS No. 30 37 688). This on the one hand increases the strength of the materials in the outer layers, and on the other hand produces useful compressive tensions in the zones adjacent the fillets, which manifest themselves in the later use of the workpieces by a considerably improved fatigue strength. These inherent compressive tensions produced in the material, however, lead inevitably to distortions in the workpieces, and the direction and amount of the distortions depend largely on the history of the workpieces. In a crankshaft these distortions result principally in misalignments of the main bearings and crankpins and hence in a distortion or "wobble" of the entire crankshaft, because in hard-rolling, inevitable deflections of the crank cheeks from the perpendicular position with respect to the crankpin axes occur. The deviations from the true running of the crankshaft with respect to its theoretical axis of rotation which this causes must be remedied by subsequent straightening of the crankshaft regardless of whether the flexing of the crankshaft cheeks by the hard-rolling is kept within defined tolerances or not. The same goes for the production of axle journals and other such workpieces having fillets, in which the distortions caused by hard-rolling also have to be remedied afterwards.
The straightening of such workpieces by bending has proven unsuitable, since the useful compressive tensions in the area of the fillets are largely relieved by the bending, so that the improved fatigue strength obtained by hard-rolling is lost.
To straighten crankshafts which have been subjected to hard-rolling it is known that this disadvantage is avoided by applying a special tool to appropriate points of the walls defining the fillets, once and with sufficient force (GB-PS No. 1,004,962, DE-PS No. 25 56 971). The directions in which these forces are to be exerted are determined by measuring the crankshaft misalignments created by the hard-rolling. In this case use is made of the knowledge that a force of sufficient magnitude acting over a limited arc of the circumference of the fillet is capable of producing permanent changes of alignment and thereby of reducing the wobbling of the crankshaft. In contrast to bending, in this straightening method only additional compressive tensions are introduced into the material, which do not reduce the fatigue strength. It is disadvantageous, however, that a special work station with an additional tool including the hydraulic press is needed for the straightening process, and great forces of up to 1000 kN must be applied to the crankshaft.
One object of this invention is, therefore, to offer a straightening method in whose application considerably lower forces will suffice. According to a further object of this invention the method shall be such that no additional tools are required. A further object is to propose an apparatus suitable for the practice of this method and for the straightening of crankshafts.